TESS Hunt for Young and Maturing Exoplanets (THYME) VI: an 11 Myr giant planet transiting a very low-mass star in Lower Centaurus Crux

2021/10/31 by Andrew W. Mann, Mackenna L. Wood, Stephen P. Schmidt +58
Physics and Astronomy · #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Astrophysics and Star Formation Studies #Exoplanet #Physics #Planet #Planetary system #Radial velocity #Stars #Stellar, planetary, and galactic studies #Transit (satellite) #astro-ph.EP #astro-ph.SR

paper · pdf · doi:10.3847/1538-3881/ac511d

published as AJ 163 156 (2022) · Accepted to the Astronomical Journal. Minor updates during referee process and proofs

arxiv created 2022/03/09 · openalex publication_date 2022/03/09 · arxiv updated 2022/03/10 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06

Abstract

Mature super-Earths and sub-Neptunes are predicted to be ≃Jovian radius when younger than 10 Myr. Thus, we expect to find 5-15R_⊕ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227 b, a 0.85±0.05R_J (9.5R_⊕) planet transiting a very low-mass star (0.170±0.015M_\odot) every 27.4 days. TOI~1227's kinematics and strong lithium absorption confirm it is a member of a previously discovered sub-group in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11±2 Myr for Musca, based on lithium, rotation, and the color-magnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of ≃0.5 M_J. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227 b is still contracting and will eventually turn into one of the more common <5R_⊕ planets.

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